Changed part of same specification but doesn't work

[Electrical_Shocker]

I replaced Phillips P80C51FA-4A with Atmel AT89C51RD2-SLSUM as the phillips one is obsolete. The specification of the micrprocessor seems to be same. Any general idea of why it won't work? I have done all the similar programming in the Atmel one.
Links are the data sheet of both

http://www.atmel.com/Images/doc4235.pdf

http://htmldatasheet.com/philips/p80c51fa4a[1].htm

 

[kellys_eye]

P80C51FA-4A.....The specification of the micrprocessor seems to be same.

is ROMLESS

....and 'seems' isn't a good enough check to establish that they are IDENTICAL....

 

[Electrical_Shocker]

Ohh got it. What would be a good part to replace Phillips one as that is obsolete?